1. Field of the Invention
The present invention relates to an imagery optical system, and more particularly to a low-priced optical lens assembly with five lens elements constituting the total length of the optical lens assembly and the imagery optical system is applied to an electronic product.
2. Description of the Related Art
As science and technology advance, the development of electronic products such as digital still cameras, web cameras and mobile phone cameras tends to have a compact design to meet the user requirements for an optical lens assembly with good aberration correction ability, high resolution, and high image quality, in addition to the compact and low-cost requirements.
In general, a conventional optical lens assembly of a mini electronic product comes with different designs, including two-lens, three-lens, four-lens, and five-or-more lens designs. If the image quality is taken into consideration, the optical lens assemblies with the four-lens and five-lens designs have advantages on aberration corrections with a better performance of their modulation transfer function (MTF), and the five-lens design having a higher resolution than the four-lens design is applicable for electronic products of high video quality or high pixel requirement.
In various mini five-lens designs of an imagery optical system with a fixed focal length, different combinations of positive and negative refractive powers are adopted. As disclosed in Japanese Pat. Publication Nos. JP2003-131136 and JP2005-266771, two sets of cemented doublets are used for reducing the total length of the optical system. As disclosed in Japanese Pat. Publication Nos. JP2003-185917 and JP2006-293042, U.S. Pat. Publication Nos. US2004/0196571 and US2003/0117722, and R.O.C. Pat. No. TW M313781, one set of cemented doublets is used to achieve a wide-angle effect. As disclosed in Japanese Pat. Publication No. JP2003-161879, the fourth-lens and the fifth-lens with different refractive powers are used to construct an optical system, but the excess total length of such optical system is not applicable for compact electronic devices.
In small products such as a digital camera, web camera, and mobile phone camera, the optical lens assembly requires a compact design, a short focal length and a good aberration correction. For different designs of the imagery optical systems with a fixed focal length and five lens elements, a better aberration correction and an appropriate total length can be achieved by utilizing the fourth and fifth lens elements with different refractive powers and an inflection point. As disclosed in R.O.C. Pat. Nos. TWM313246, TW201038966, TW201022714 and TWM332199 and U.S. Pat. No. 7,710,665, a good aberration correction is achieved, but the total length of the optical system still cannot meet the requirements of compact electronic devices. As disclosed in U.S. Pat. No. 7,826,151, US2010/0254029 and US2010/0253829, the fourth lens element and the fifth lens element having an inflection point each are adopted for achieving a shorter total length of the optical system, but these patents disclose the fourth lens element and the fifth lens element with inflection points to correct the aberration or image distortion, and thus a longer gap exists between the third lens element and the fourth lens element which is not favorable in achieving a shorter total length. Furthermore, excess variation of the fourth lens element brings the fabrication lots of difficulties and is unfavorable for the cost reduction. Therefore, the present invention provides a feasible design to shorten the optical lens assembly, while using the refractive powers of the five lens elements with the combination of convex and concave surfaces to improve the image quality for the application on compact electronic devices and simplify the complexity of the manufacturing, in addition to the effect of reducing the total length of the optical lens assembly.